1. Field of the Invention
The present invention relates to packages and sealing techniques, and more particularly, to hermetic seals, hermetically sealed semiconductor packages, and hermetic sealing techniques.
2. Discussion of the Related Art
Semiconductor packages have proven useful in protecting integrated circuits and in attaching those circuits to electrical boards. More recently, such semiconductor packages have proven useful in protecting and mounting devices such as Micro-Electro Mechanical Systems (MEMS) assemblies and other imaging devices.
Semiconductor packages protect integrated circuits from harm caused by humidity, harmful gases, and various materials used in electronic assembly. Such protection usually takes the form of an airtight environmental seal, referred to herein as a hermetic seal. However, hermetic seals are used in many more applications than environmentally sealing semiconductor packages. Therefore, a hermetic seal as used herein refers to any environmental seal of any enclosed space.
Hermetically sealing semiconductor packages presents various challenges. For example, a semiconductor package might be subjected to a wide range of temperatures and thermal differentials, vibrations, accelerations, impacts, loads, and radiations. Compounding the sealing challenge is the relatively large number of individual hermetic seals that can be required. To understand this, consider that an integrated circuit (or another device) must be mounted in the package. Thus, a seal for an access port (usually a lid) is required. Further consider that an integrated circuit must electrically connect to an external circuitry, and that electrically conductive leads or pads, possibly a very large number of such leads or pads, must pass through the semiconductor package. Each of those leads or pads requires a hermetic seal. Another challenge is that in some applications, such as micro-mirror assemblies and UV erasable memories, light must pass through the semiconductor package. Still another challenge is hermetically sealing at high speed and at low cost.
Prior art semiconductor packages are generally comprised of a body having an embedded set of conductive leads and a cavity for holding a protected device. Electrical bonding wires electrically interconnect the conductive leads and the protected device. A lid is then placed over the cavity's access port. That lid is usually attached to the body using an adhesive such as an epoxy. Additionally, the locations where the conductive leads enter the body are usually sealed with a sealing compound, often an epoxy. The lid adhesive and sealing compound are typically applied using a liquid dispenser. Time, heat, and/or UV light is used to cure the dispensed liquid.
Examples of prior art semiconductor packages include Dual In-Line Packages (DIP), Plastic Pin Grid Array packages (PPGA), and Small Outline Integrated Circuit (SOIC) packages. For cost considerations, the body and lid are usually thermoplastic. However, ceramic semiconductor packages are sometimes used, particularly when light must pass through the semiconductor package. Then, a ceramic body and a ceramic lid having a transparent glass window are beneficial.
While generally successful, prior art semiconductor packages have certain problems. For example, bonding the lid to the body and sealing the conductive leads using a dispensed liquid are time consuming and can lead to contamination. Also, the required handling steps when applying the adhesive and/or sealing compound can lead to physical damage to the package or to the packaged device. Furthermore, many adhesives and sealants introduce moisture into the cavity, possibly leading to device corrosion and failure, or to hermetic seal cracks caused by thermally induced pressure during soldering or operation of the protected device. Indeed, any difference in the coefficients of thermal expansion of any of the parts can induce stresses and/or strains that can lead to hermetic seal failure.
Modern high-density integrated circuits (ICs) having a large number of connections place additional stresses on prior art hermetic seal techniques. For example, some integrated circuits require well over one hundred pins. Hermetically sealing such a large number of pins is difficult and time consuming as careful pin placement and alignment becomes increasingly critical and difficult, as does ensuring a reliable hermetic seal.
One prior art approach to hermetic sealing uses ultrasonic welding. Ultrasonic welding refers to welding using an intense, controlled vibration that develops friction-induced heating between elements. This heating, combined with pressure, causes melting of one or both of the welded elements at the interface between those elements. Upon solidification, a mechanical bond is created between the elements. According to the prior art, when ultrasonic welding is used with semiconductor packages, frictional heat is developed by placing a lid on a body, applying a predetermined pressure between the lid and the body, and then applying an intense ultrasonic vibration to the lid. The intense vibration is transmitted through the lid to induce a welding temperature. The induced temperature causes a flow of melted material that produces a semi-hermetic or hermetic seal between the lid and the body.
With prior art ultrasonic welding of semiconductor packages, a thermoplastic body and lid are preferred. Thermoplastic materials are common, inexpensive, and ultrasonically weld readily. Furthermore, thermoplastics are typically resilient, have only a relatively small amount of outgassing, and can have matching thermal coefficients of expansion.
While ultrasonic welding of semiconductor packages is useful, prior art ultrasonic welding techniques do not work well when attempting to hermetically seal brittle materials, such as ceramics and glass. Direct ultrasonic welding of those materials, even to a thermoplastic body, tends to induce cracks, breaks, and other damage to the brittle material.
Therefore, a new type of hermetic seal would be beneficial. Even more beneficial would be a new type of hermetic seal that is useful for hermetically sealing semiconductor packages. In particular, a new type of hermetic seal capable of sealing conductive pins to a body, and/or a lid to a body would be useful. Furthermore, a new hermetically sealed semiconductor package would also be beneficial. Also beneficial would be a new method of producing hermetic seals and a new way of assembling semiconductor packages.